Method and apparatus for radar turbulence detection

ABSTRACT

Method and apparatus for detecting atmospheric turbulence with a conventional, non-Doppler radar by detecting echo signals from two pulse volumes radially spaced apart along the radar beam. The measurement of the atmospheric turbulence is effected by measuring the average power of the echo signal from each of the two pulse volumes, by measuring the variance in the fluctuation spectra of these signals, and by measuring the variance of the signals representative of the predetection sum of the echo signals from both pulse volumes. The difference in the mean velocities of the scatterers in the pulse volumes, a measurement of the atmospheric turbulence, can be calculated since it is a function of the difference between the power-weighted variance of the fluctuation spectrum of the predetection sum of the echo signals from both pulse volumes and the sum of the power-weighted variances of the fluctuation spectra of the echo signals from each of the pulse volumes.

Waited States Patent Atlas [54] METHOD AND APPARATUS FOR RADARTURBULENCE DETECTION David Atlas, 5522 South Harper, Chicago, Ill. 60637[22] Filed: May 2,1969

[21] Appl.No.: 821,389

[72] Inventor:

Primary ExaminerT. H. Tubbesing Attorney-Dressler, Goldsmith, Clement &Gordon RADAR BEAM Feb.29,1972

[5 7] ABSTRACT Method and apparatus for detecting atmospheric turbulencewith a conventional, non-Doppler radar by detecting echo signals fromtwo pulse volumes radially spaced apart along the radar beam. Themeasurement of the atmospheric turbulence is effected by measuring theaverage power of the echo signal from each of the two pulse volumes, bymeasuring the variance in the fluctuation spectra of these signals, andby measuring the variance of the signals representative of thepredctection sum of the echo signals from both pulse volumes. Thedifference in the mean velocities of the scatterers in the pulsevolumes, a measurement of the atmospheric turbulence, can be calculatedsince it is a function of the difference between the power-weightedvariance of the fluctuation spectrum of the predetection sum of the echosignals from both pulse volumes and the sum of the power-weightedvariances of the fluctuation spectra of the echo signals from each ofthe pulse volumes.

25 Claims, 8 Drawing Figures PULSE VOLUMES IPAIENTEIIFEBZS m2 SHEET 1[IF 3 M a o. 0 A W 4 5 "a M E J J U B M .m R R 5 R0 Rm RA m LA 0 M 4 0 Io 9 6 F H P T W M 'x IIF II R W IA D I PULSE VOLUMES I I RADIAL AIRVELOCITY, V

INVENTOR.

DAV/D ATLAS wgzimub ATTORNEYS.

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METHOD AND APPARATUS FOR RADAR TURBULENCE DETECTION BACKGROUND Thedetection and measurement of atmospheric turbulence is not only ofconsiderable academic interest but is a problem of considerablepractical importance in meteorology and in aviation.

Thus, the detection and measurement of atmospheric turbulence is ofgreat concern in the aviation industry in order to allow aircraft tobecome aware of, and avoid where possible, turbulence which appears tobe hazardous or capable of discomforting passengers riding the aircraft.Such turbulence may take the form of storms containing reflectingscatterers such as rain, snow or hail or may be associated with thephenomenon of clear air turbulence. Such clear air turbulence may bedetected either by virtue of associated refractive index perturbationsor by means of radar reflecting chaff dispersed into such regions, forexample, by aircraft, balloons or other suitable means.

It is of course possible to use a Doppler radar to measure relativemotion between the radar source and the target, such as particles in astorm. Fluctuations in this relative velocity as a function of distancethrough a storm are due to disturbances in horizontal wind. Thus, itwould be relatively simple to measure turbulence with a Doppler radar.

Unfortunately, existing airborne weather radar systems are not Dopplerradars and do not have Doppler capabilities, and most ground-basedweather radars are similarly deficient. Conventional (non-Doppler)radars are, however, in general use in all transport aircraft and asground radars for detection and mapping of storms.

Present techniques for assessing the potential hazard of a detectedstorm involve the measurement of signal intensity or storm reflectivity.This approach is based on the assumption that the reflectivity isproportional to the number and size of the scatterers, e.g., rain, snowor hail; and an increase in reflectivity is attributable to an increasein these quantities, i.e., a greater precipitation rate, and, therefore,greater associated updrafts and downdrafts. In other words, thisapproach is based on the assumption that the more reflective the storm,the more severe are its associated flight hazards.

Refinements in this approach utilize a gradient of reflectivity or thesharpness with which the reflectivity varies from point to point in astorm. Again this technique is based on the theory that this gradient isrelated to turbulence intensity.

Unfortunately, neither of these assumptions-that the magnitude ofreflectivity or the gradient of reflectivity are related to turbulenceintensity-is generally valid. They are merely qualitative associationswhich appear to fail under a variety of conditions.

The purpose of detecting turbulence intensity is to determine or measurethe velocity perturbations which will be experienced by an aircraftwhich traverses the turbulent region. This means that what is requiredis a measurement of vertical air movement or vertical air perturbationsas a function of distance along a flight path.

Where this measurement is not feasible, an alternative is to measureperturbations in horizontal air motion along the flight path on theassumption that the air turbulence is isotropic, i.e., independent ofdirection. Even if the turbulence is not isotropic, it appears that theintensity of the perturbations in horizontal air motion is generallyclosely correlated with perturbations in vertical air motion.

Thus, if it is possible to measure the horizontal air motion and theperturbations therein, it would be possible to have a good indication ofthe vertical air motion and perturbations therein, and, therefore, anindication of the atmospheric turbulence which has the greatest adverseeffect and causes the most concern in aviation.

As indicated above, the measurement of such turbulence with a Dopplerradar would be a relatively simple procedure since Doppler radar has thecapability of detecting differences LII measure the movement of thescatterers forming a part of the atmosphere.

SUMMARY OF INVENTION In accordance with the present invention, there isprovided a method and apparatus for utilizing conventional, non-Dopplerradar for the measurement and detection of atmospheric turbulence. Themeasurement and detection of atmospheric turbulence is effected by thedetection of echo signals from two pulse volumes radially spaced apartalong the radar beam and by the measurement of the average power andfluctuation spectral characteristics of the echo signals from each ofthese two separate pulse volumes and by a corresponding measurement ofthe spectral characteristics of a signal representing the predetectionsum of the echo signals from the two volumes.

Since the variance of this sum channel is a function of thepower-weighted sum of the variance of the fluctuation spectra in theindependent channels, plus a term dependent upon the difference in meanradial velocity of the scatterers in the associated pulse volumes, thedifferences in the mean radial velocity of the reflectors in each volumecan be measured. When the spacing between the sample volumes is adjustedto represent distance along an aircraft flight path, for example, thismeasurement is a measure of the turbulence to be expected in traversingthe space between these pulse volumes.

More specifically, the reflected signals from a radar beam may beprocessed to extract information from the reflected signalsrepresentative of specific locations along the radar beam about whichinformation is desired. This is effected by gating the receiver to beresponsive to only those portions of the received signals which arebeing reflected from the volumes in space located at the desired radialposition along the beam.

In order to obtain meaningful information regarding atmosphericturbulence, signals reflected from two radially spaced apart volumes areprocessed in conjunction with each other. A conventional radar isresponsive primarily to the time varying intensities of the signalsreflected from the scatterers within each and every pulse volume. Thisinformation is utilized to obtain the difference in the mean velocitiesin the two volumes, a measure of the turbulence intensity at scalescorresponding to the spacing between the two volumes, and smaller.

Thus, detected video signals at the output of conventional radar systemsmay be range gated at two ranges with a selectable radial spacingbetween the two ranges. The range gated outputs represent theinstantaneous echo intensities received from the two radially spacedpulse volumes.

The time variations in the intensity of the signals received from eachof the pair of pulse volumes are analyzed to provide a measure of theaverage echo power and the root-meansquare signal fluctuation rate orvariance. Simultaneously, signals received from each of the pair ofpulse volumes are combined prior to detection and the time variation ofthe signal intensity of this signal is also analyzed to provide ameasure of its average echo power and root-mean-square fluctuation rateor variance. 1

The signals from the two pulse volumes can be combined, for example, bysumming them prior to detection in the LP. or R.F. stages of the radarreceiver. This can be accomplished by means of a delay line having adelay corresponding to the spacing between the pair of pulse volumes.

When the mean radial velocity of the scatterers in the two individualpulse volumes differ, the spectral characteristics of the fluctuationspectrum corresponding to the detected output of the combined signalsdiffer from the spectral characteristics of the fluctuation spectracorresponding to the signals reflected from the individual pulsevolumes. This difference is utilized as a measure of the difference inthe mean radial velocity of the scatterers in the respective pulsevolumes, and thus as a measure of the intensity of the turbulence to beexpected in traversing the distance between the two volumes.

A series of echo signals received from a first volume are processed togenerate the average power of the echo signals from the first pulsevolume and the variance of the fluctuation spectrum of such signals.Similarly the average power and variance of the fluctuation spectrum ofecho signals from a second volume can similarly be measured. Inaddition, the signals reflected from both pulse volumes are summedbefore detection and the variance of the fluctuation spectrum of thecombined echo signals is obtained. With this information, the differencein the mean velocities of the scatterers in the two volumes can beobtained in accordance with the following equation:

V, mean radial velocity of the scatterers in a first pulse volume r,mean radial velocity of the scatterers in a second pulse volume itwavelength P, [I average power or intensity of the echo signals from thefirst pulse volume P QT) average power or intensity of the echo signalsfrom the second pulse volume 2, variance of the fluctuation spectrum ofthe echo signals from the first volume 2 variance of the fluctuationspectrum of the echo signals from the second volume 2. variance of thefluctuation spectrum of the predetection sum of the echo signals fromthe first and second pulse volumes Thus equation I) shows that thedifference in the mean velocities of two radially spaced apart pulsevolumes is related to the difference between the power-weighted varianceof the fluctuation spectrum of the predetection sum of the echo signalsand the sum of the power-weighted variances of the fluctuation spectraof the echo signals from each of the pulse volumes.

In use as airborne radar in which the aircraft might very well beapproaching the storm, it is of course desirable to repetitively samplesignals from the same volumes of air within the storm throughout theduration of the measurement. This is effected by detecting the leadingedge of the storm and gating the reflected signals into the measuringcircuitry with respect to the leading edge of the storm regardless ofaircraft position relative to the storm.

Thus, by the present invention, the characteristics of the fluctuationspectra of the radar signals reflected from a pair of selected volumesrelatively fixed in space can be processed by use of a conventionalradar to provide information regarding atmospheric turbulence and tomeasure the turbulence that can be expected in traversing the spacebetween the two volumes.

It should be understood that since in accordance with the presentinvention the method and apparatus for detecting atmospheric turbulenceeffectively provides a measure of the difference in the mean radialvelocity of the scatterers between two preselected ranges, the systemand method may also provide a measure of the radial wind shear. Thus,for example, when a radar beam is directed essentially horizontally, astrong radial wind shear may be indicative of the existence of a sharpfrontal surface or tornado or other disturbance.

The significance of this capability takes on meaning when consideringthe hazards of aircraft takeoff or landings. Along the takeoff orlanding profile of an aircraft, strong wind shear may be hazardous tosafe takeoffs or landings and, therefore, the system and method of thepresent invention is capable of providing a measurement and warning ofpotentially hazardous wind shear. Similarly, the disclosed system andmethod may also be used to indicate the presence of potential presenceof tornados since these conditions are commonly associated with sharpradial variations in mean wind velocity.

Numerous other advantages and features of the present invention willbecome readily apparent from the following detailed description of theinvention and of one embodiment thereof, from the claims and from theaccompanying drawings in which each and every detail shown is fully andcompletely disclosed as a part of this specification, in which likenumerals refer to like parts.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a diagrammatic view showingthe use of a radar from both a moving and a fixed station;

FIG. 2a is a diagrammatic view of a radar beam traversing pulse volumesin a storm;

FIG. 2b is a diagrammatic view of a signal showing the variation ofradial velocity as a function of range;

FIG. 3 is a block diagram of one embodiment of a radar turbulencemeasurement system;

FIG. 4 is a block diagram of an alternative embodiment of a radarturbulence measurement system;

FIG. 4a is an alternative of FIG. 4;

FIG. 5 is a block diagram of a subsystem suitable for use in the systemof FIG. 3; and

FIG. 6 is another alternative embodiment of a radar turbulencemeasurement system.

DETAILED DESCRIPTION Referring to FIGS. 1, 2a and 2b, conventional radardetection of atmospheric turbulence is initiated by directing a radarbeam 10 from a source such as an aircraft 12 or a ground station 14toward a region of scatterers 16 such as a storm cloud l8, precipitationparticles or artificial tracers such as radar chaff which may have beendispersed into the air to provide detectable echoes.

It can be appreciated that each of the scatterers 16 moves at varyingradial velocities. FIG. 2b is a schematic trace 17 of the beam-averagedradial velocities of the scatterers as a function of distance along theradar beam 10 disclosed in FIG. 20.

At any instant in time, the echo returned to the radar receivercorresponds to a volume of space 20 encompassed with the radar beam andhaving a radial depth along the beam equal to h/2 where h is the lengthof the transmitted pulse. If t is the time duration of the transmittedpulse, h=ct where c is the speed of light.

The effective pulse depth in space from which echoes are receivedsimultaneously is h/2 because when the trailing edge of the pulse isbeing reflected from particles at range r, the leading edge of the pulsewill have had time to reach and return from scatterers at a range r+h/2from the radar. Accordingly, the signal received at any instant is thesum of the echoes from all scatterers within a range depth h/2 in thedetectable region.

As indicated diagrammatically in FIG. 20 there are a great manyscatterers 16 within each pulse volume. In Doppler radar the netdetected echo signal amplitude from a single pulse volume is the sum ofeach of the echo signal amplitudes reflected at the various Dopplerfrequencies.

When this signal is detected coherently and passed through a spectrumanalyzer, we obtain the so-called Doppler spectrum corresponding to thesignal or the average power returned as a function of Doppler frequencyshift. If P is the total average power returned by all the scatterersand Smdf is the fraction of that power which is returned at Dopplerfrequencies between f and f+df, then PS(f)df is the Doppler spectrum.

Because f=2vllt where A is wavelength and v is radial velocity of thescatterers, the Doppler spectrum is an image of the radial velocityspectrum or the distribution of echo power with radial velocity.Clearly, from the Doppler spectrum as measured with a Doppler radar, themean Doppler shift and corresponding mean radial velocity as well as thebreadth or variance of the spectrum around that mean velocity can bedetermined.

If this is done at two radially spaced ranges, corresponding t o twopulse volumes, then the mean Doppler frequency shifts f and f2, and theassociated mean radial velocities V. and F, can also be obtained. Thedifference (ti-7 or its square, is a measure of the turbulence to beexperienced in traversing the distance between the two pulse volumes.

Unfortunately, this cannot be done with a conventional, non-Dopplerradar. In such a radar system, the signals are detected without acoherent reference; typically by a square law envelope detector whoseoutput is proportional to I(t) the instantaneous square of the signalamplitude A(t). Neglecting the oscillating term corresponding to thecarrier frequency, A(t) may be represented by:

A(t)='2 z,eiwir (2) where a, may be considered to represent the signalamplitude returned by all scatterers moving with radial velocity v, andproducing Doppler shift f, and angular Doppler frequency and:

i= fi= w tll- (3) Thus, the instantaneous output of the envelopedetector of the conventional radar may be expressed by:

i l m i C05 r fl 1 (4) it can be seen from Equation (4) that the signalintensity is comprised of a DC term I( t)= a? corresponding to theaverage intensity, and fluctuating components at angular rates:

u=(fl-fl)= r j) (6) In other words, the beating of the signals from allthe scatterers having radial velocity v, and Doppler shift f, with thosehaving velocity v,- and Doppler shift f, produces fluctuations of thesignal intensity with frequency F =(f f,). If the signal having anintensity I(t) represented by Equation (4) is passed through a spectrumanalyzer, the result is the so-called fluctuation spectrum." It may beshown readily that the fluctuation spectrum:

This is the integral of the Doppler spectrum convolved with itself afterbeing displaced by fluctuation frequency F. Since F takes on bothpositive and negative values and S(F)=S(F), 1 and:

Fleisher in "R-meter: an Instrument for Measuring Gustiness" M.l.'l.Dept. of Meteorology, Weather Radar Research Report No. 24, i955).

lf the signals returned from the first pulse volume 20 are added tovthose from the second pulse volume 22 in the intermediate frequency (IF)or radiofrequency stages prior to detection, the sum signal aftercoherent phase detection (in a Doppler radar) would be:

A(t)=2a,, cos 21r(fl;7)t-l-Ed,, cos 21r(f, f )t 9 where the individualDoppler sQfts in e ach volume relative to the mean Doppler frequencies,f, and f in respective volumes are indicated. The net Doppler spectrumis the sum of the individual Doppler spectra, each weighted according toits total power:

P, l,(t) the total power in the first spectrum, i.e., the average powerof the signals reflected from a first pulse volume P 32'l (t) the totalpower in the second spectrum, i.e., the average power of the signalsreflected from a second pulse volume S.(f) normalized spectrum of thesignals reflected from a first pulse volume S (f) normalized spectrum ofthe signals reflected from a second pulse volume S, (f) normalizedspectrum of the predetection sum of the signals reflected from the firstand second pulse volumes where s ffidfil W i th a Doppler radar, theindividual mean frequencies f, and f in the respective pulse volumes andthe combined spectrum of the two volumes could be measured and thus ii-Tdetermined. This or its square is the desired measurement of turbulence,or, in the case of organized motions, a measure of the radial shearbetween the two volumes.

However, with the incoherent radar, only the fluctuation spectrum ofsignal intensities can be measured; either the entire fluctuationspectrum, or, for example, only its variance, see Equation (8), where itis shown that the variance of the fluctuation spectrum is exactly twicethat of the Doppler spectrum. Thus, the variance of the combined Dopplerspectrum in terms of the variance of the individual spectra from the twovolumes should first be determined. It is readily shown that:

(l1) and The variance of the spectrum is given by the equation:

By combining Equations (ll) and (12) the variance of the combinedDoppler spectra can be obtained:

However, an incoherent radar can measure only the variances of theassociated fluctuation spectra. Thus, noting that 0 /2 and thatFZv/A, itcan be shown that:

It thus can be seen that measurement of the average power and varianceof the fluctuation spectra for each of the two volumes separately andthe variance of the spectrum of the combined volumes will generatesufficient information to provide an output indicative of the turbulenceintensity or radial shear at scales corresponding to the space inbetween the two volumes and at smaller scales.

This spacing may be chosen so as to correspond to the turbulent scalesby which aircraft are most severely affected. Alternatively, the spacingL may be varied progressively so that ifi=i (r) and v =i7(r+L), where ris the distance to volume 1 and (rl-L) is the distance to volume 2.

The quantity D(L) =[v(r)v(r-l-i'.,] is the so-called structure functionof turbulence and is related quantitatively to the "u ns rum:

turbulence spectrum. In particular D(L) is a close approximation to thekinetic energy in the turbulence spectrum at scales equal to and smallerthan scale L. Accordingly, the difference of measurements at two scalesL, and L., or D(L )D(L,), where L L., provides a close approximation tothe turbulent kinetic energy at scales between L. and L Referring now toFIG. 3 there is shown a block diagram for one system suitable forgenerating an output indicative of such atmospheric turbulence. In thissystem the echo signals on line 24 are amplified by [.F. amplifier 26.The instantaneous output of IF amplifier 26 represents the sum of all ofthe echo signals received at that instant from a radar pulse volume.

The output 28 of IF. amplifier 26 is connected to two channels, a singlevolume channel 30, and a sum channel 32. Output 28 is connected to boththe single channel 30 and to the sum channel 32. Output 28 is alsoconnected to the sum channel 32 through an LP. delay 34 which delays thesignal by an amount FL/Zc where L is the desired spacing of the twovolumes between which the turbulence measurement is to be made and c isthe speed of light.

The output 28 from the LF. amplifier 26 is connected to the input of adetector 40 in the single volume channel 30. The undelayed output 28 andthe output 36 from the LP. delay 34 are added in an IF. adder 38 theoutput 42 of which is connected to the input of a detector 44 in sumchannel 32. Thus, the output 36 from [.F. delay 34 at time tworepresents the signals actually received at time one, i.e., from thefirst pulse volume 20, while the output 28 of the [.F. amplifier 26 attime two represents the signals received at time two from the secondpulse volume 22.

it is to be understood, that the echo signals can be separated into asingle volume channel and a sum channel at any stage in the system priorto detection. As can be seen in FIG. 4, for example, echo signals online 45 are delayed in an RF. delay 46 the output of which is connectedto an R.F. adder 47. The signals on line 45 are also connected directlyto the R.F. adder 47. The output 48 of RF adder 47 represents thepredetection sum of the signals received from the first pulse volume andthe second pulse volume 22. The output 48 of the RF adder and thesignals on line 45 are each amplified in separate R.F. amplifiers 49,are passed through separate mixers 50, amplified in I.F. amplifiers 26',the outputs of which are connected, respectively, to detectors 44 and40.

The mixers 50 mix the respective input R.F. signals with a signal from aconventional local oscillator (not shown) in order to heterodyne theradiofrequencies down to intermediate frequencies (IF) capable of beingpassed by the IF. amplifiers, all as is well known.

Alternatively, as shown in FIG. 4a, the signals on line 45 can beinitially amplified in a single R.F. amplifier 49' before being splitinto the single volume and sum channels. The output of RF. amplifier 49is delayed in RP. delay 46, the output of which is connected to an RFadder 47'. The output of the RF. amplifier 49 is also connected to theRF adder 47, the output of which is again representative of thepredetection sum of the signals from the first and second pulse volumes.As in FIG. 4, the output of the RF adder 47' and of the R.F. amplifier49 are separately processed through mixers 50 and [.F. amplifiers 26'before being inputted to the detectors 44 and 40, respectively.

One output 52 of the single volume detector 40 is connected to power andfluctuation spectrum variance circuits 53. 54 for the first and secondvolumes, respectively. A second output 55 from the detector 40 isconnected to a storm-leading edge trigger 56 which generates triggerpulses representative ofthe leading edges of each reflecting region orstorm.

The output 57 of trigger 56 can be selected, for example, by manualoperation. to correspond to a storm to be measured, and thus generatestrigger pulses only for the selected storm. These trigger pulses permitthe subsequent circuitry to operate in response only to the selectedstorm.

The output 57 from storm leading edge trigger 56 controls a variablepulse delay circuit 58 which gates the output 52 of detector 40 into therespective power and variance circuits 53, 54 so that each of thesecircuits receives signals only from the respective pulse volumes 20, 22.Variable pulse delay circuit 58 may gate the detector circuit 40 so thatit responds only to signals from the desired pulse volumes and may inaddition or alternatively gate each of the power and variance circuitsto respond to only that portion of the output from detector 40corresponding to the representative pulse volumes.

Thus, variable pulse delay circuit 58 performs two functions. Itcontrols the delay between the storm leading edge trigger pulse and thefirst gating pulse, thereby allowing for selection of the desiredposition of the first pulse volume relative to the edge of the storm.The variable pulse delay circuit also controls and varies the delaybetween the first gating pulse and the second gating pulse to vary thedistance L between the pulse volumes. In this manner, the region withina storm to be examined can be selected and the distance between twopulse volumes can be set to any desired scale of turbulence e.g., to thescale to which an aircraft is most sensitive.

The output 60 of the sum channel detector 44 is connected to a sum powerand variance circuit 62 which also is gated by delay circuit 58 to limitthe input from the sum channel detector to a period of time during whichthe output is representative of the sum of the signals from the firstand second pulse volumes 20,22.

In the case of a ground based radar, storm leading edge trigger 56 isnot generally required since the measurements of power and variance canbe made at a selected range fixed with respect to the radar. Only whenthe radar is on a moving platform is it required to provide a referencetrigger at the leading edge of the storm so that the succession of echopulses required for power and variance measurements will have beenreturned from the same pulse volume throughout the measurement period.

In the case of airborne radars, the variable pulse delay circuit 58 andthe LF. delay 34 can be automatically linked to the aircraft's airspeedindicator, thus setting the turbulence scale L in accordance with thatto which the aircraft is most sensitive at the speed in question.

Each of the power and variance circuits S3, S4 and 62 generate an outputrepresentative of the average power and the fluctuation spectrumvariance, respectively, corresponding to the first pulse volume 20, thesecond pulse volume 22, and the predetection sum of the signals receivedfrom both pulse volumes 20, 22. These outputs are connected to anarithmetic unit 64 which performs the computations required by Equation(1) to generate an output representative of the difference between themean radial velocity in the two volumes and therefore the turbulenceintensity at scales equal to and smaller than the spacing between thetwo volumes.

It should be realized, of course, that successive or simultaneousmeasurements may be accomplished not only between the first and secondpulse volumes 20, 22, but between other pulse volumes, thus displayingturbulence intensity at all ranges in the atmospheric region underobservation. It should be also apparent that such a system may makemeasurements between two pulse volumes which are spaced apart byincreasing steps of L. This then would provide an indication of theturbulence structure function and spectrum as a function of the spacingbetween pulse volumes.

FIG. 5 shows one embodiment of a power and variance circuit for use inthe system of FIG. 3. Thus, the output from a detector 40 is range gatedby variable delay 58 and is sampled and held by a boxcar circuit 66 theoutput 68 of which is integrated by an integrator 70 having a timeconstant adjusted to the desired sample length. The output 72 of theintegrator 70 is the average power at the selected range. The averagepower is one of the parameters required in the computation of turbulenceintensity.

The average power output 72 is also used to set a threshold level inthreshold level circuit 74 to a value equal to or proportional to theaverage power. A frequency meter 76 measures nun: nAnn the rate at whichthis level is crossed in one direction by the individual echo signalintensities. The output 78 of the frequency meter is theroot-mean-square fluctuation frequency 2, the standard deviation of thefluctuation spectrum. This output is squared in the squaring circuit 80to provide an output directly proportional to the variance, the otherparameter required in the computation of turbulence intensity.

Although a plurality of power and variance circuits are shown in FIG. 3,it should be clear that a single circuit can be used sequentially ifappropriate storage capabilities are inserted into the system. It isevident also that the gating and power and variance measurementcircuitry of FIG. 3 may be duplicated any number of times to providesimultaneous turbulence measurements at a multiplicity of ranges throughthe storm.

In some circumstances, it is of course desirable to measure and displayturbulence simultaneously at all ranges throughout a storm. Since atypical storm may contain some 100 to 200 or more pulse volume pairs, itwould become expensive and would require excessive equipment volume toreproduce the gating and power and variance measurement circuitry ofFIG. 3. To overcome this restriction, the single channel video and sumchannel signal may be stored in the manner illustrated by the embodimentof FIG. 6, which has the capability of obtaining measurements at aplurality of ranges throughout the storm.

In such a multiple range system, the output 28 of the LF. amplifier 26is again connected to two channels. After detection in detector 40, thesuccessive pulses are converted to a digital form in A/D converter 82and stored in a digital memory 86. Each row 88 of the memory 86 storespulses from different volumes in range for a single echo signal and eachcolumn 90 stores successive echo signals from the same pulse volume. Ina similar manner, the output of detector 44 of the sum channel issimilarly converted in an A/D converter 82 and stored in memory 86 in asimilar arrangement.

After the receipt of all of the echo signals, the average power of thesignals stored in each range column 90 can be obtained by reading outthe stored information in each column 90 into averager 92 and averagingthese outputs. The output 94 from averager 92 for each column 90 isstored in buffer memory 96. The stored value of average echo power 98 isthen used to set a threshold level circuit 100, the threshold levelbeing changed sequentially as the read out of memory 86 is stepped fromrange to range.

The signals for each range are again read out in time sequence and thecounter 102 may then count the total number of times that the change inintensity from one time to the other crosses the average power level setin threshold circuit 100 in a unipolar direction. This number isproportional to the r.m.s. frequency of the fluctuation spectrum, andthus to its standard deviation. This calculation can be repeated foreach range and stored in the buffer memory 104.

The output of the two buffers 96, 104 are connected to an arithmeticunit 64 which performs all the operations required to generate an output106 representative of (V!'V2 for each and every pair of pulse volumes,the signals from which are being analyzed. The output 106 may bereconverted to an analog form by a D/A converter 108 and displayed toprovide a graphic presentation of turbulence as a function of distancealong the radar beam in display 110.

The output may also be used to trigger an alarm 112 if the outputexceeds a preselected threshold level 114 indicating that the stormshould not be penetrated.

It should be realized that the various functions performed by thesystems disclosed in FIGS. 3, 4, 4a, and 6 may also be accomplished byvarious other techniques. For example, average echo power can bemeasured by use of either analog or digital integrators or bymeasurement of the integral of the entire fluctuation spectrum ofsignals received from a given pulse volume. The variance of thefluctuation spectrum can alternatively be obtained by employing aspectrum analyzer to measure the entire fluctuation spectrum andcomputing its variance automatically.

Where storage of the received data is desired, it is clear that it maybe stored either in analog or digital form and that in either case thedesired measurements and computations can be performed by use of eitherdigital or analog circuitry or a combination thereof. It should also beunderstood that the functions disclosed as occurring in the IF. stagesof the system could also be accomplished in the RF. stages with themodification of the associated circuitry.

Thus, there has been disclosed a radar system and method for measurementof atmospheric turbulence without the utilization of Doppler techniques.This capability permits the acquisition of desired atmosphericinformation by use of generally less complex circuitry and at low cost.Of great significance is the fact that such capabilities can beincorporated as a part of existing radar systems thereby overcoming oneof the major objections to other proposals, the need to replaceexpensive equipment already in use.

From the foregoing, it will be observed that numerous'variations andmodifications may be effected without departing from the true spirit andscope of the novel concept of the invention. It is, of course, intendedto cover by the appended claims all such modifications as fall withinthe scope of the claims.

What is claimed is:

1. A method for detecting atmospheric turbulence comprising the steps ofradiating a radar beam towards a region of scatterers, receiving echosignals reflected from said scatterers including first echo signalsreflected from scatterers located in a first pulse volume along saidbeam and second echo signals reflected from scatterers located in asecond pulse volume along said beam spaced radially from said firstpulse volume, combining said first and second echo signals prior todetection to generate combined echo signals, sensing the average powerof said first echo signals, said second echo signals and said combinedecho signals and generating power level signals representative thereof,sensing the spectral characteristics of the fluctuation spectra of saidfirst echo signals, said second echo signals and said combined echosignals and generating spectra signals representative thereof, andoperating on said power level and spectra signals to provide as afunction of the average power and the spectra characteristics of saidfirst echo signals, said second echo signals and said combined echosignals intelligence representative of the difference between the meanradial velocity of the scatterers in said first pulse volume and themean radial velocity of the scatterers in said second pulse volume as anindication of the atmospheric turbulence between said pulse volumes.

2. A method as claimed in claim 1 wherein said first and second echosignals are received sequentially, and in which the step of combiningsaid first and second echo signals includes the step of delaying saidfirst echo signals until said second echo signals are received andadding said first and second signals together prior to detection togenerate said combined echo signals.

3. The method as claimed in claim 2 including the step of varying thedelay of said first echo signals to alter the spacing between said pulsevolumes.

4. A method as claimed in claim 3 including the steps of receiving andcombining said echo signals sequentially, storing said signals as afunction of range and time until a selected number of said signals havebeen received, sensing the average power and spectral characteristics ofthe fluctuation spectra of the received and combined echo signals ofselected pairs of pulse volumes, whereby intelligence indicative of theturbulence at selected radial positions along said radar beam can beprovided.

5. A method as claimed in claim 2 including the steps of radiating saidradar beam from a moving aircraft and varying said delay as a functionof the speed of said aircraft.

6. A method as claimed in claim 5 including the steps of generating atrigger signal representative of the leading edge of said scatterers andgating said echo signals relative to said trigger signal, whereby saidfirst pulse volume is spaced radially a selected distance from theleading edge of said scatterers.

Ill.

1'. A method as claimed in claim 2 including the steps of adrustablygating said echo signals to maintain said pulse volumes fixed in spaceduring the measurement period independent of aircraft velocity.

3. A method as claimed in claim 1 including the steps of receiving saidecho signals from a plurality of different pulse volumes spaced radiallyfrom each other, combining different pairs of said received echo signalsprior to detection to generate a plurality of different combined echosignals, repeating the remaining steps to provide said intelligencerepresentative of the difference between the mean radial velocity ofscatterers in one pulse volume of each pair of pulse volumes and themean radial velocity of the scatterers of the other pulse volume in eachpair of pulse volumes to provide an indication of the atmosphericturbulence as a function of range.

9. A method as claimed in claim 8 including the step of displaying saidintelligence as a function of range.

10. A method as claimed in claim 8 including the steps of combining saidreceived echo signals from a first and a second volume prior todetection to generate a first combined echo signal, combining saidreceived echo signals from said first and a third pulse volume prior todetection to generate a second combined echo signal, repeating theremaining steps to provide said intelligence representative of thedifference between the mean radial velocity of scatterers in said firstpulse volume and the mean radial velocity of the scatterers in saidsecond pulse volume and to provide said intelligence representative ofthe difference between the mean radial velocity of scatterers in saidfirst pulse volume and the mean radial velocity of the scatterers insaid third pulse volume to provide an indication of the atmosphericturbulence as a function of range between each pair of pulse volumes,whereby a comparison of said indications of atmospheric turbulence maybe made to provide an approximation of the turbulent kinetic energytherebetween.

ll. A method as claimed in claim 1 including the step of varying thespacing between said first and second pulse volumes.

12. A method as claimed in claim 1 in which the step of sensing thespectral characteristics of said first echo signals, said second echosignals and said combined echo signals includes the steps of measuringthe frequency variance of the fluctuation spectra of each of said firstecho signals, said second echo signals and said combined echo signals.

i3. A method as claimed in claim 12 including the step of providing saidintelligence as a function of the difference between the power-weightedvariance of the fluctuation spectrum of said combined echo signals andthe sum of the powerweighted variances of the fluctuation spectra ofsaid first echo signals and said second echo signals.

14. A method as claimed in claim 1 in which the step of sensing theaverage power of said first echo signals, said second echo signals andsaid combined echo signals includes the steps of detecting the intensityof each of said first echo signals, measuring the average power of saidfirst echo signals and generating in response thereto first power levelsignals representative of the average power of said first echo signals,detecting the intensity of each of said second echo signals, measuringthe average power of said second echo signals and generating in responsethereto second power level signals representative of the average powerof said second echo signals and detecting the intensity of each of saidcombined echo signals, measuring the average power of said combined echosignals and generating in response thereto combined power level signalsrepresentative of the average power of said combined echo signals, inwhich the step of sensing the spectral characteristics of said firstecho signals, said second echo signals and said combined echo signalsincludes the steps of measuring the frequency variance of thefluctuation spectrum of said first echo signals and generating a firstvariance signal representative thereof, measuring the frequency varianceof the fluctuation spectrum of said second echo signals and generating asecond variance signal representative thereof and measuring thefrequency variance of the fluctuation spectrum of said combined echosignals and generating a combined variance signal representative thereofand including the steps of operating on said first, second and combinedaverage power signals and said first, second and combined variancesignals to provide said intelligence in accordance with the followingfor- :mula:

P )]where V, average mean radial velocity of the scatterers in a firstpulse volume 17 mean radial velocity of the scatterers in a second pulsevolume A wavelength P, TR?) average power or intensity of the echosignals from the first pulse volume P m average power or intensity ofthe echo signals from the second pulse volume I, variance of thefluctuation spectrum of the echo signals from the first volume 2variance of the fluctuation spectrum of the echo signals from the secondvolume E variance of the fluctuation spectrum of the combined echosignals from the first and second pulse volumes 15. A method as claimedin claim 14 in which the step of measuring the variance of each of saidfirst echo signals, said second echo signals and said combined echosignals includes the step of measuring the rate at which the individualecho signal intensities of each of the respective echo signals cross athreshold level proportional to the average power of the correspondingecho signals.

16. A method as claimed in claim 1 including the steps of radiating saidradar beam from a moving aircraft and varying the spacing between saidfirst and second pulse volumes as a function of the speed of saidaircraft, and selecting that delay corresponding to the scale ofturbulence to which the aircraft is most sensitive at the correspondingair speed.

17. A method for detecting atmospheric turbulence by determining thedifference between the mean radial velocity of scatterers in one of apair of pulse volumes and the mean radial velocity of scatterers inanother of said pair of pulse volumes comprising the steps of radiatinga radar beam towards a region of scatterers, receiving echo signalsreflected from said scatterers, operating on said echo signals to definethe echo signals reflected from each of a plurality of pulse volumesspaced radially apart along said beam, sensing the average power of eachof the echo signals reflected from a pair of said pulse volumes andgenerating power level signals representative thereof, sensing thespectral characteristics of each of said echo signals reflected fromsaid pair of pulse volumes and generating spectra signals representativethereof, combining said echo signals reflected from said pair of pulsevolumes prior to detection to generate combined echo signals, sensingthe average power of said combined echo signals and generating combinedpower level signals representative thereof. sensing the spectralcharacteristics of said combined echo signals and generating combinedspectra signals representative thereof, and operating on said powerlevel and spectra signals to provide as a function of the average powerand spectral characteristics of each of said echo signals intelligencerepresentative of said difference in the mean velocity of scatterers inthe pair pulse volumes as an indication of the atmospheric turbulencebetween said pulse volumes.

18. A method as claimed in claim 17 including, the steps of receivingand combining said echo signals received from different pairs of pulsevolumes sequentially, storing said signals as a function of range andtime until a selected number of said signals have been received, sensingthe average power and spectral characteristics of the fluctuationspectra of the received and combined echo signals from selected pairs ofpulse volumes, to provide intelligence representative of said difierencein the mean velocity of scatterers in each of said different pairs ofpulse volumes as an indication of atmospheric turbulence between each ofsaid plurality of different pairs of pulse volumes, and providing anindication of atmospheric turbulence as a function of radial positionalong said radar beam.

19. A radar system for detection of atmospheric disturbance comprising aradar transmitter, a radar antenna coupled to said transmitter forradiating a radar beam towards a region of scatterers and for receivingecho signals reflected from said scatterers, said echo signals includingfirst echo signals reflected from scatterers located in a first pulsevolume along said beam and second echo signals reflected from scattererslocated in a second pulse volume along said beam spaced radially fromsaid first pulse volume, first circuit means receiving said echo signalsfrom said source and combining said first and second echo signals inresponse thereto, second circuit means receiving said echo signals fromsaid source and said first circuit means and generating power levelsignals representative of the average power of said first, second andcombined echo signals in response thereto, third circuit meansresponsive to said first, second and combined echo signals for measuringthe spectral characteristics thereof and generating spectra signalsrepresentative thereof, and output circuit means receiving said powerlevel and spectra signals and generating in response thereto an outputrepresentative of the difference between the mean radial velocity of thescatterers in said first pulse volume and the mean radial velocity ofthe scatterers in said second pulse volume as an indication of theatmospheric turbulence between said pulse volumes.

20. A system as claimed in claim 19 including range gating circuit meansfor limiting the input to said first, second and third circuit means tosaid first and second echo signals.

21. A system as claimed in claim 19 wherein said third circuit meansincludes variance measuring circuit means receiving said first, secondand combined echo signals and said power level signals and generating avariance signal representative of the variance of the fluctuationspectra of said first, second and combined echo signals in responsethereto; said output circuit means including means receiving said powerlevel and variance signals and computing the difference between thepower-weighted variance of the fluctuation spectrum of said combinedecho signals and the sum of the powerweighted variance of thefluctuation spectra of said first and second echo signals, and forgenerating an output representative thereof.

22. A system as claimed in claim 19 wherein said first circuit meansincludes an LP. delay receiving said first echo signals and delayingsaid first echo signals an amount equal to the spacing between saidpulse volumes, and an [.F. adder receiving the delayed first echosignals and said second echo signals simultaneously and combining saidsignals by adding said signals together prior to detection.

23. A system as claimed in claim 19 wherein said first circuit meansincludes an RF. delay receiving said first echo signals and delayingsaid first echo signals an amount equal to the spacing between saidpulse volumes, and an R.F. adder receiving the delayed first echosignals and said second echo signals simultaneously and combining saidsignals by adding said signals together prior to detection.

24. A system as claimed in claim 19 including a threshold circuit, meansfor setting said threshold circuit to a preselected value of atmosphericturbulence between said first and second pulse volumes indicative ofhazardous turbulence intensity, and an alarm circuit connected to saidthreshold circuit and said output circuit means and generating an alarmin response to an output from said circuit means indicative ofatmospheric turbulence between said pulse volumes greater than saidpreselected level.

- 25. A radar system for detection of atmospheric turbulence at aplurality of ranges throughout the radial extent of a region ofatmospheric scatterers comprising a radar source for radiating a radarbeam towards said region of scatterers and for receiving echo signalsreflected from said scatterers, said echo signals including separateecho signals reflected from scatterers located in each of a plurality ofpulse volumes spaced radially along said beam, first circuit meansreceiving the echo signals from each of said pulse volumes andgenerating an output representative thereof, first detector circuitmeans receiving said first circuit output and generating a singledetection signal for each signal received from each pulse volume, delaycircuit means receiving said first circuit output and delaying saidoutput a selected time interval, signal adding circuit means receivingthe said delayed output and said first circuit output and addingtogether said outputs generating a combined echo signal for each of thesignals received from each of a pair of pulse volumes, second detectioncircuit means receiving said combined echo signals and generating acombined detection signal for each combined signal from each pair ofpulse. volumes, signal storage means receiving said single detectionsignals and said combined detection signals and separately storing eachof said detection signals, second cir cuit means extracting storedsingle and combined detection signals from said signal storage means foreach pair of pulse volumes and generating power level signalsrepresentative of the average power of said single and combined echosignals for each pair of pulse volumes in response thereto, thirdcircuit means extracting said stored single and combined detectionsignals from said signal storage means for each pair of pulse volumesand measuring the spectral characteristics of the fluctuation spectrathereof and generating spectral signals representative thereof, andoutput circuit means receiving said power level and spectral signals andgenerating in response thereto outputs representative of the differencebetween the mean radial velocity of the scatterers in one pulse volumeand the mean radial velocity of the scatterers in the other pulse volumefor each pair of pulse volumes and providing an output indicative of theatmospheric turbulence as a function of range throughout said region ofscatterers.

It-mu- Air- UNITED STA'IES PATENT OFFICE QERTIFICATE OF CORRECTIONPatent No. 3,846,555 Dated February 29- 1972 Inven fl David. Atlas It iscertified that error appears in the above-identified patent and thatseid Letters Patent are hereby corrected as shown below:

(3011mm equation (1), that port on of the equation reading '2 2 P AYEshould read 2 line #1, 5 should read M2152 Column line 52, "with" shouldread within.

Column 5 equation (2), that 1, which reads e e iw should read ng M;

equation 4); that porfiion oi the equation reading "2 a 22 should read-Za 22".

column 6, equation (9), that portion of the equation reading "(f 13should read fig f line 21, "P 32 T 3 0 F" should read --P TQKEL? 2equation (1 4), those eortione of the equation reading "(P 1 and "(P a2P and '/(P P should read --(P r and. (P a) and --=-/(P Pg) respectively.

Col mg 12 line 7 that portio of the formula reading "(V1 m V2) 9! sllOuld l$ a fig j Column 13 lines 6 and 7, "transmitter, a radar antennacoupled to said transmitter" should read source--.,

Signed and sealed this 18th day of July 1972,

iISEAL) Attestfi EDWARD MQFLETGHER J'RQ ROBERT GOTTSCHALK AttestlngOfficer Gommissioner of Patents FORM PO'WSO than, Uscomm-OC mam-mu) o s(JIVIRHHIIII rnmhvm mllrl I'M! \u-qu UNITED STATES PATENT GFFICE'CERTIFICATE OF CORRECTION Patent 3,64 ,555 Dated February 29. 1972Invent r( David Atlas It is certified that error appears in theabove-identified patent and that said Letters Patent are herebycorrected as shown below:

Column 3, equation (1), that portion of the equation reading "Z 2 P AYT"should read 2 Pan"; v

n 2" I line 41, 2' should read 2 Column 4, line 52, "with" should read--within--.

Column 5 equation (2), that per n which reads a a e iw should read" ia ev equation (4),- that portion oi' the equation reading '2 3. 22 shouldread Za 22-".

Column 6, equation (9), that portion of the equation reading "(f 1'should read 1' line 21, "P 32 1 W)" should read "P 2 equation (1'4),those nortions oi the equation v reading (P l and "(P az P and '/(P Pshould read "-(P1f1 and --(P and --/(P P2) respectively. 3

Col 12 line 7 that portio oi the formula reading "(V if?? should readColumn 13 lines 6 and 7, "transmitter, a radar antenna ooupled to saidtransmitter" should read "source-n Signed and sealed this 18th day ofJuly 1972.

iiSEAL) Attest.

EDWARD M.FLETCHER,J'R. ROBERT GOTTSCHALK attesting Officer Commissionerof Patents FORM PO-OSO (10-69) USCOMM-DC flOIHO-PGQ v u 5 (minimumrnnunzr. mun mu r- \u' an

1. A method for detecting atmospheric turbulence comprising the steps ofradiating a radar beam towards a region of scatterers, receiving echosignals reflected from said scatterers including first echo signalsreflected from scatterers located in a first pulse volume along saidbeam and second echo signals reflected from scatterers located in asecond pulse volume along said beam spaced radially from said firstpulse volume, combining said first and second echo signals prior todetection to generate combined echo signals, sensing the average powerof said first echo signals, said second echo signals and said combinedecho signals and generating power level signals representative thereof,sensing the spectral characteristics of the fluctuation spectra of saidfirst echo signals, said second echo signals and said combined echosignals and generating spectra signals representative thereof, andoperating on said power level and spectra signals to provide as afunction of the average power and the spectra characteristics of saidfirst echo signals, said second echo signals and said combined echosignals intelligence representative of the difference between the meanradial velocity of the scatterers in said first pulse volume and themean radial velocity of the scatterers in said second pulse volume as anindication of the atmospheric turbulence between said pulse volumes. 2.A method as claimed in claim 1 wherein said first and second echosignals are received sequentially, and in which the step of combiningsaid first and second echo signals includes the step of delaying saidfirst echo signals until said second echo signals are received andadding said first and second signals together prior to detection togenerate said combined echo signals.
 3. The method as claimed in claim 2including the step of varying the delay of said first echo signals toalter the spacing between said pulse volumes.
 4. A method as claimed inclaim 3 including the steps of receiving and combining said echo signalssequentially, storing said signals as a function of range and time untila selected number of said signals have been received, sensing theaverage power and spectral characteristics of the fluctuation spectra ofthe received and combined echo signals of selected pairs of pulsevolumes, whereby intelligence indicative of the turbulence at selectedradial positions along said radar beam can be provided.
 5. A method asclaimed in claim 2 including the steps of radiating said radar beam froma moving aircraft and varying said delay as a function of the speed ofsaid aircraft.
 6. A method as claimed in claim 5 including the steps ofgenerating a trigger signal representative of the leading edge of saidscatterers and gating said echo signals relative to said trigger signal,whereby said first pulse volume is spaced radially a selected distancefrom the leading edge of said scatterers.
 7. A method as claimed inclaim 2 including the steps of adjustably gating said echo signals tomaintain said pulse volumes fixed in space during the measurement periodindependent of aircraft velocity.
 8. A method as claimed in claim 1including the steps of receiving said echo signals from a plurality ofdifferent pulse volumes spaced radialLy from each other, combiningdifferent pairs of said received echo signals prior to detection togenerate a plurality of different combined echo signals, repeating theremaining steps to provide said intelligence representative of thedifference between the mean radial velocity of scatterers in one pulsevolume of each pair of pulse volumes and the mean radial velocity of thescatterers of the other pulse volume in each pair of pulse volumes toprovide an indication of the atmospheric turbulence as a function ofrange.
 9. A method as claimed in claim 8 including the step ofdisplaying said intelligence as a function of range.
 10. A method asclaimed in claim 8 including the steps of combining said received echosignals from a first and a second volume prior to detection to generatea first combined echo signal, combining said received echo signals fromsaid first and a third pulse volume prior to detection to generate asecond combined echo signal, repeating the remaining steps to providesaid intelligence representative of the difference between the meanradial velocity of scatterers in said first pulse volume and the meanradial velocity of the scatterers in said second pulse volume and toprovide said intelligence representative of the difference between themean radial velocity of scatterers in said first pulse volume and themean radial velocity of the scatterers in said third pulse volume toprovide an indication of the atmospheric turbulence as a function ofrange between each pair of pulse volumes, whereby a comparison of saidindications of atmospheric turbulence may be made to provide anapproximation of the turbulent kinetic energy therebetween.
 11. A methodas claimed in claim 1 including the step of varying the spacing betweensaid first and second pulse volumes.
 12. A method as claimed in claim 1in which the step of sensing the spectral characteristics of said firstecho signals, said second echo signals and said combined echo signalsincludes the steps of measuring the frequency variance of thefluctuation spectra of each of said first echo signals, said second echosignals and said combined echo signals.
 13. A method as claimed in claim12 including the step of providing said intelligence as a function ofthe difference between the power-weighted variance of the fluctuationspectrum of said combined echo signals and the sum of the power-weightedvariances of the fluctuation spectra of said first echo signals and saidsecond echo signals.
 14. A method as claimed in claim 1 in which thestep of sensing the average power of said first echo signals, saidsecond echo signals and said combined echo signals includes the steps ofdetecting the intensity of each of said first echo signals, measuringthe average power of said first echo signals and generating in responsethereto first power level signals representative of the average power ofsaid first echo signals, detecting the intensity of each of said secondecho signals, measuring the average power of said second echo signalsand generating in response thereto second power level signalsrepresentative of the average power of said second echo signals anddetecting the intensity of each of said combined echo signals, measuringthe average power of said combined echo signals and generating inresponse thereto combined power level signals representative of theaverage power of said combined echo signals, in which the step ofsensing the spectral characteristics of said first echo signals, saidsecond echo signals and said combined echo signals includes the steps ofmeasuring the frequency variance of the fluctuation spectrum of saidfirst echo signals and generating a first variance signal representativethereof, measuring the frequency variance of the fluctuation spectrum ofsaid second echo signals and generating a second variance signalrepresentative thereof and measuring the frequency variance of thefluctuation spectrum of said combined echo signals and generating acombined variance signal Representative thereof and including the stepsof operating on said first, second and combined average power signalsand said first, second and combined variance signals to provide saidintelligence in accordance with the following formula: (V1-v2)2 ((lambda 2/8) (P1+P2)/P1P2)( Sigma 1,22(P1+P2)-( Sigma 12P1+ Sigma 22P2))where v1 average mean radial velocity of the scatterers in a first pulsevolume v2 mean radial velocity of the scatterers in a second pulsevolume lambda wavelength P1 I1(t) average power or intensity of the echosignals from the first pulse volume P2 I2(t) average power or intensityof the echo signals from the second pulse volume Sigma 12 variance ofthe fluctuation spectrum of the echo signals from the first volume Sigma22 variance of the fluctuation spectrum of the echo signals from thesecond volume Sigma 1,22 variance of the fluctuation spectrum of thecombined echo signals from the first and second pulse volumes
 15. Amethod as claimed in claim 14 in which the step of measuring thevariance of each of said first echo signals, said second echo signalsand said combined echo signals includes the step of measuring the rateat which the individual echo signal intensities of each of therespective echo signals cross a threshold level proportional to theaverage power of the corresponding echo signals.
 16. A method as claimedin claim 1 including the steps of radiating said radar beam from amoving aircraft and varying the spacing between said first and secondpulse volumes as a function of the speed of said aircraft, and selectingthat delay corresponding to the scale of turbulence to which theaircraft is most sensitive at the corresponding air speed.
 17. A methodfor detecting atmospheric turbulence by determining the differencebetween the mean radial velocity of scatterers in one of a pair of pulsevolumes and the mean radial velocity of scatterers in another of saidpair of pulse volumes comprising the steps of radiating a radar beamtowards a region of scatterers, receiving echo signals reflected fromsaid scatterers, operating on said echo signals to define the echosignals reflected from each of a plurality of pulse volumes spacedradially apart along said beam, sensing the average power of each of theecho signals reflected from a pair of said pulse volumes and generatingpower level signals representative thereof, sensing the spectralcharacteristics of each of said echo signals reflected from said pair ofpulse volumes and generating spectra signals representative thereof,combining said echo signals reflected from said pair of pulse volumesprior to detection to generate combined echo signals, sensing theaverage power of said combined echo signals and generating combinedpower level signals representative thereof, sensing the spectralcharacteristics of said combined echo signals and generating combinedspectra signals representative thereof, and operating on said powerlevel and spectra signals to provide as a function of the average powerand spectral characteristics of each of said echo signals intelligencerepresentative of said difference in the mean velocity of scatterers inthe pair pulse volumes as an indication of the atmospheric turbulencebetween said pulse volumes.
 18. A method as claimed in claim 17including, the steps of receiving and combining said echo signalsreceived from different pairs of pulse volumes sequentially, storingsaid signals as a function of range and time until a selected number ofsaid signals have been received, sensing the average power and spectralcharacteristics of the fluctuation spectra of the received and combinedecho signals from selected pairs Of pulse volumes, to provideintelligence representative of said difference in the mean velocity ofscatterers in each of said different pairs of pulse volumes as anindication of atmospheric turbulence between each of said plurality ofdifferent pairs of pulse volumes, and providing an indication ofatmospheric turbulence as a function of radial position along said radarbeam.
 19. A radar system for detection of atmospheric disturbancecomprising a radar transmitter, a radar antenna coupled to saidtransmitter for radiating a radar beam towards a region of scatterersand for receiving echo signals reflected from said scatterers, said echosignals including first echo signals reflected from scatterers locatedin a first pulse volume along said beam and second echo signalsreflected from scatterers located in a second pulse volume along saidbeam spaced radially from said first pulse volume, first circuit meansreceiving said echo signals from said source and combining said firstand second echo signals in response thereto, second circuit meansreceiving said echo signals from said source and said first circuitmeans and generating power level signals representative of the averagepower of said first, second and combined echo signals in responsethereto, third circuit means responsive to said first, second andcombined echo signals for measuring the spectral characteristics thereofand generating spectra signals representative thereof, and outputcircuit means receiving said power level and spectra signals andgenerating in response thereto an output representative of thedifference between the mean radial velocity of the scatterers in saidfirst pulse volume and the mean radial velocity of the scatterers insaid second pulse volume as an indication of the atmospheric turbulencebetween said pulse volumes.
 20. A system as claimed in claim 19including range gating circuit means for limiting the input to saidfirst, second and third circuit means to said first and second echosignals.
 21. A system as claimed in claim 19 wherein said third circuitmeans includes variance measuring circuit means receiving said first,second and combined echo signals and said power level signals andgenerating a variance signal representative of the variance of thefluctuation spectra of said first, second and combined echo signals inresponse thereto; said output circuit means including means receivingsaid power level and variance signals and computing the differencebetween the power-weighted variance of the fluctuation spectrum of saidcombined echo signals and the sum of the power-weighted variance of thefluctuation spectra of said first and second echo signals, and forgenerating an output representative thereof.
 22. A system as claimed inclaim 19 wherein said first circuit means includes an I.F. delayreceiving said first echo signals and delaying said first echo signalsan amount equal to the spacing between said pulse volumes, and an I.F.adder receiving the delayed first echo signals and said second echosignals simultaneously and combining said signals by adding said signalstogether prior to detection.
 23. A system as claimed in claim 19 whereinsaid first circuit means includes an R.F. delay receiving said firstecho signals and delaying said first echo signals an amount equal to thespacing between said pulse volumes, and an R.F. adder receiving thedelayed first echo signals and said second echo signals simultaneouslyand combining said signals by adding said signals together prior todetection.
 24. A system as claimed in claim 19 including a thresholdcircuit, means for setting said threshold circuit to a preselected valueof atmospheric turbulence between said first and second pulse volumesindicative of hazardous turbulence intensity, and an alarm circuitconnected to said threshold circuit and said output circuit means andgenerating an alarm in response to an output from said circuit meansindicative of atmospheric turbulence between said pulse voluMes greaterthan said preselected level.
 25. A radar system for detection ofatmospheric turbulence at a plurality of ranges throughout the radialextent of a region of atmospheric scatterers comprising a radar sourcefor radiating a radar beam towards said region of scatterers and forreceiving echo signals reflected from said scatterers, said echo signalsincluding separate echo signals reflected from scatterers located ineach of a plurality of pulse volumes spaced radially along said beam,first circuit means receiving the echo signals from each of said pulsevolumes and generating an output representative thereof, first detectorcircuit means receiving said first circuit output and generating asingle detection signal for each signal received from each pulse volume,delay circuit means receiving said first circuit output and delayingsaid output a selected time interval, signal adding circuit meansreceiving the said delayed output and said first circuit output andadding together said outputs generating a combined echo signal for eachof the signals received from each of a pair of pulse volumes, seconddetection circuit means receiving said combined echo signals andgenerating a combined detection signal for each combined signal fromeach pair of pulse volumes, signal storage means receiving said singledetection signals and said combined detection signals and separatelystoring each of said detection signals, second circuit means extractingstored single and combined detection signals from said signal storagemeans for each pair of pulse volumes and generating power level signalsrepresentative of the average power of said single and combined echosignals for each pair of pulse volumes in response thereto, thirdcircuit means extracting said stored single and combined detectionsignals from said signal storage means for each pair of pulse volumesand measuring the spectral characteristics of the fluctuation spectrathereof and generating spectral signals representative thereof, andoutput circuit means receiving said power level and spectral signals andgenerating in response thereto outputs representative of the differencebetween the mean radial velocity of the scatterers in one pulse volumeand the mean radial velocity of the scatterers in the other pulse volumefor each pair of pulse volumes and providing an output indicative of theatmospheric turbulence as a function of range throughout said region ofscatterers.